1. Field of the Invention
This invention relates to a misfire-detecting system for a multiple cylinder-type internal combustion engines which detects a misfire, i.e. an abnormal firing occurring in a cylinder when spark ignition is not properly effected in the cylinder, and more particularly to a misfire-detecting system which is adapted to detect a misfire based on variations or fluctuations in the rotational speed of the crankshaft of the engine.
2. Prior Art
Recently, in order to cope with the problem of environmental pollution, etc., there has been an increasing need for detecting a misfire occurring in an internal combustion engine, which causes emission of undesirably rich exhaust gases. Further, it is also required to determine which cylinder is suffering from a misfire, particularly in an internal combustion engine for automotive vehicles, which is of a multiple cylinder type.
A misfire-detecting system of this type has been disclosed in Japanese Provisional Patent Publication (Kokai) No. 2-112646, in which a misfire is detected based on variation in the rotational speed of the engine. More specifically, in this system, the rotational speed of the crankshaft is momentarily detected when a piston within each cylinder is in a particular position, e.g. in the vicinity of a top dead center position, and a misfire is detected based on variation in the rotational speed of the crankshaft thus detected.
Further, a misfire-detecting system for internal combustion engines has also been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-49955, in which a signal is output whenever the crankshaft rotates through a predetermined angle, and variation in the time intervals between signals thus output (variation in the rotational speed of the crankshaft) is detected to thereby detect a cylinder in which a misfire is occurring, based on the variation in the rotational speed of the crankshaft.
In general, the rotational speed of the engine (i.e. the rotational speed of the crankshaft) has a periodically changing characteristic when momentarily monitored with a very short time span such that it assumes the lowest value when the piston is close to its top dead center position, and then higher values until the next piston becomes close to its top dead center position. More specifically, in a multiple cylinder-type engine, while a certain cylinder is in the compression stroke, its piston reaches its top dead center position, and then the cylinder enters the explosion stroke where a compressed air-fuel mixture within the cylinder is ignited, so that the rotation of the crankshaft increases because the crankshaft is accelerated by downward movement of the piston under the pressure of the explosion gas. Then, the rotational speed of the crankshaft progressively decreases due to resistance applied by load on the engine until the next cylinder is subjected to ignition, which again causes acceleration of the crankshaft. This process is repeatedly carried out. Therefore, the rotational speed of the engine has a periodically changing characteristic, as mentioned above.
However, when a misfire occurs in one of the cylinders, the acceleration of the rotation of the crankshaft is not effected by the cylinder, so that the rotational speed continues to decrease even after its piston passes the top dead center position.
The above-mentioned proposed misfire-detecting systems utilize this fact, by detecting a momentary rotational speed of the crankshaft when each cylinder is in the vicinity of its top dead center position to thereby monitor variation in the rotational speed of the crankshaft, and by determining that the cylinder is normal when the variation is small, whereas a misfire has occurred in the cylinder when the variation is large, i.e. the rotational speed of the crankshaft has largely decreased. Further, it is possible to determine the cylinder in which a misfire has occurred, by identifying the cylinder which just passed the top dead center position immediately before a large decrease in the rotational speed was detected.
However, in the proposed misfire-detecting systems, a signal indicative of the engine rotational speed at a particular position of each cylinder is directly used for misfire detection, which makes it impossible to determine whether a variation in the rotational speed has been caused by a misfire or by another factor.
More specifically, the signal indicative of the detected engine rotational speed reflects a variation in the rotational speed which occurs at a frequency twice as high as the rotational speed of the engine if the engine is a four-cylinder type, and at a frequency three times the rotational speed of the engine if the engine is a six-cylinder type. Further, an internal combustion engine installed on an automotive vehicle also undergoes variations in the rotational speed which are caused by changes in the condition in which the vehicle is operating, such as acceleration or deceleration of the vehicle and travelling on a rough road. Therefore, the signal indicative of the detected rotational speed contains variations in the rotational speed which are caused by various factors other than a misfire.
As a result, if the signal indicative of the engine rotational speed is directly used for misfire detection as in the proposed system, the signal can contain noise produced particularly when the engine is in a high rotational speed/low load condition or when the engine is in a relatively low rotational speed condition while the vehicle is travelling on a rough road, which makes it difficult to discriminate a variation in the rotational speed which is caused by a misfire, resulting in low accuracy of misfire detection and difficulty in determining the cylinder which is suffering from the misfire.
The present inventors have made intensive studies to solve these problems, and found that by sequentially detecting a momentary rotational speed of the engine to obtain a waveform signal, and causing the signal to pass through a filter to obtain a signal having a particular frequency component therefrom, this signal contains considerably reduced variations in the rotational speed of the engine caused by factors other than a misfire. By monitoring the signal having a particular frequency component, it is possible to clearly and distinctly determine a variation in the rotational speed caused by a misfire, which enables to improve the accuracy of misfire detection.
However, when the engine undergoes a misfire, the rotational speed of the engine, after having decreased as described above, rises again due to a phenomenon of so-called "reactionary motion" of the main body of the engine and then falls again, followed by repeating this rise and fall process or vibration. The frequency of the reactionary motion is smaller than the frequency of variation in the rotational speed caused by a misfire but fairly close to the latter. Therefore, it is difficult to eliminate the influence of the reactionary motion by merely causing the signal to pass a filter. To eliminate the influence of the reactionary motion, it is necessary to increase the filtering frequency of the filter for filtering off low frequency signal components. If the filtering frequency is thus increased, however, the signal indicative of a decrease in the rotational speed caused by a misfire per se diminishes, and moreover vibration caused by the reactionary motion is not made smaller relative to the degree of diminution of the signal, which results in the possibility that a decrease in the rotational speed caused by the reactionary motion is erroneously taken for one caused by a misfire.
The reactionary motion is liable to occur when the engine rotational speed is low, whereas when the engine rotational speed is high, the signal indicative of the crankshaft rotational speed used for detecting variation in the crankshaft rotational speed is liable to contain noise produced by torsional vibration of the crankshaft or vibration thereof due to so-called "shaky movement" of journals, which also makes it difficult to accurately detect a misfire.